Pumping systems which are used to pump drilling mud and other abrasive fluids generally incorporate positive displacement pistons or plungers which operate in a reciprocating manner in individual cylinders. Each piston or plunger cylinder has a suction and discharge valve and valve seat operating alternatingly and independently to control flow into and out of each cylinder. The valve typically comprises a disc-shaped body portion which includes an elastomeric valve insert which serves to seal the valve when in the closed position and also serves to cushion the impact of the valve body in the valve seat. The lower conical seating surface of the valve body also makes contact with the valve seat when the valve is in the closed position.
Early prior art valve assembly designs included a steel valve body and steel valve seat that were both heat treated to increase wear resistance. However, heat treating usually caused warpage in the sealing surface, and thus a rubber insert was typically used to help seal between irregular warped steel valve bodies and the valve seat surfaces. The rubber insert also helped to seal around solid particles in the mud that were trapped between the valve seat. These valves provided acceptable performance at low pressures, but the rubber insert failed frequently due to extrusion and abrasion damage. The insert generally required replacement up to 10 times over the life of the steel valve body and steel valve seat.
Later, circa 1965, a new valve assembly design referred to as the TRW Mission Design was introduced. This design incorporated a 4-web carburized valve seat that became an industry standard. The design also incorporated an elastomer insert on the valve body. The elastomer insert was manufactured from a polyurethane material having a 90-shore A durometer hardness, a high strength modulus, and excellent abrasion and extrusion resistance compared to prior rubber inserts. Use of polyurethane inserts raised the effective valve pressure rating of the valve assembly. These valve assemblies required less frequent changing of inserts, and the 90-shore A durometer hardness also became an industry standard. The few disadvantages to these polyurethane inserts were that the hard 90-shore A material had a very low resilience, provided poor sealability in worn valve seats and/or unprimed pumps, and had less abrasion resistance compared to softer polyurethanes. An example of a metal valve body having an elastomeric seal is described in U.S. Pat. No. 4,860,995 to Rogers.
Because of the above disadvantages of harder polyurethane, such as the 90-shore A durometer, engineers would have preferred using a softer polyurethane. However, this was not possible because, concurrent with this design, average drilling pressures increased due to ever deeper drilling. Also this design introduced heat treatment by carburizing a process that increased steel hardness and wear resistance, but also increased warpage. The harder polyurethane was also necessary because of the increased warpage of the steel valve bodies during the heat treat carburizing manufacturing process. This warpage resulted in extrusion gaps between the mating valve body and valve seat which has caused premature failure of softer polyurethanes.
Pumping systems which are used to pump abrasive fluids undergo tremendous pressure and valve bodies and seats therefore suffer impact and stress high wear. One of the most significant points of impact stress in the valve assembly is the point of contact of the valve body and the valve seat when the valve is in the fully closed position. The extremely high differential pressures in the valve cause the valve body to engage the valve seat with a very high impact. The repetitive impact eventually causes the valve body to become worn and fatigued, necessitating its replacement. Because of high pump pressures (between 1000 and 5000 psi) and the abrasive solid particles suspended in the drilling mud, prior art valve bodies and valve seats were required to be constructed of steel that was both heat treated and carburized to increase durability.
However, the manufacture of steel valve assemblies is generally a lengthy and expensive process. First, it is necessary to procure raw material in the form of forgings or castings, which then must be machined and heat treated. In many instances, it is then necessary to again machine these elements and, in some designs, further heat treat them. The length of time required for the total manufacturing operation is generally six months or more. The manufacturing cost is also considerable.
Although various types of plastic materials have been available for years or even decades, the industry has generally taught against the use of plastic valve bodies in high pressure applications because of the assumption that plastic lacked the strength and durability at these pressures. The few attempts to design a plastic valve for high pressure applications have failed. For example, U.S. Pat. No. 5,062,452 to Johnson discloses a valve member comprising a lower support portion and an upper seal portion. The support portion is made of a relatively rigid plastic,a hard cast polyurethane, and the seal portion is made of a relatively soft plastic, a softer grade of polyurethane. The Johnson patent mentions both oil well and water well applications for this valve. However, testing has proven that this valve is unusable in oil well applications.
One problem in using urethane as taught in Johnson is that these plastics generally lack rigidity. Further, these plastics are temperature sensitive and lose what rigidity they have very quickly as temperature increases. As a result, the valves described in the Johnson patent fail at high pressures, and thus can only be used in very low pressure applications. In addition, Johnson teaches that the valve member is made of cast urethane using an open molded process. This process is very labor and time intensive and thus considerably increases the cost of the assembly.
Thus, there has been a long-felt need for a new valve body design with strength and durability similar to that of steel valves but with reduced manufacturing time and cost. Also, there has been a long-felt need for a valve body design which allows use of more desirable softer polyurethane inserts for improved sealability, resilience and abrasion resistance.